Convector surface cooking unit

ABSTRACT

An air convection range surface heater has a centrifugal air impeller for recirculating electrically heated air and includes fixed stator vanes operative with the impeller to increase the mass airflow through a heat exchanger and into an air circulation space adjacent a utensil to thereby distribute maximum heat flux to the utensil. The unit provides both inner and outer cooperating spill chambers that readily separate the heated air from spillage to eliminate fire and electrical short circuiting hazards. A solid state air monitor and control circuit is provided for throttling power input to limit the maximum air temperature in the unit&#39;&#39;s heat exchanger at any desired value.

United States Patent James R. llornaday, Jr.

[72] Inventors Troy; Edwin J. Miller, Mt. Clemens; Charles W. Vigor, Rochester, all of Mich. [21 Appl. No. 22,149 [22] Filed Mar. 24, 1970 [45] Patented Jan. 4, 1972 [73] Assignee General Motors Corporation Detroit, Mich.

[541 CONVECTOR SURFACE COOKING UNIT 13 Claims, 4 Drawing Figs.

[52] U.S. Cl. 219/370, 219/368, 219/369, 219/374 [51] Int. Cl F24h 3/04 [50] FieldofSearch 219/366, 367, 368, 370, 374, 375, 376, 369

[56] References Cited UNITED STATES PATENTS 1,313,258 8/1919 Carrnean 219/370 Primary Examiner-J. V. Truhe Assistant Examiner-Gale R. Peterson v Attorneys-Frederick M. Ritchie, William S. Pettigrew and Edward 1. Barthel ABSTRACT: An air convection range surface heater has a centrifugal air impeller for recirculating electrically heated air and includes fixed stator vanes operative with the impeller to increase the mass airflow through a heat exchanger and into an air circulation space adjacent a utensil to thereby distribute maximum heat flux to the utensil. The unit provides both inner and outer cooperating spill chambers that readily separate the heated air from spillage to eliminate fire and electrical short circuiting hazards, A solid state air monitor and control circuit is provided for throttling power input to limit the maximum air temperature in the unit's heat exchanger at any desired value.

PATENTED JAM 4l972 SHEET 2 0 3 7 A d N Q av I N VE NTORS.

Jane; K (Va/2706159 f6 a! (:25. 0 A T TOR N F y PATENTEB JAN 4 ER SHEET 3 [IF 3' CONVECTOR SURFACE COOKING UNIT This invention relates to a domestic appliance and more particularly to an improved electric range surface unit operating on the principal of convection heat transfer.

The use of convection heating in a surface unit of an electric range is broadly disclosed and claimed in applicants US. Pat. No. 3,167,638 entitled Surface Cooking Unit" issued Jan. 26, 1965 to J. R. Homaday, Jr., et al. and assigned to the same assignee as the present application. Reference may be had to that patent for the broad principles underlying the present invention.

The present invention, although utilizing some of the teachings shown in that prior disclosure, contemplates a convector air-type surface heating unit having an improved recirculating air system, means for separating the heated air from spilled material the provision of a novel control circuit for limiting the maximum air temperature. Also applicants discovered a novel arrangement whereby the airflow rate through the surface unit could be improved by a design that increases the pressure of the air and eliminates undesirable air swirl prior to its entry into the heat exchange portion of the unit.

Accordingly it is an object of this invention to provide a convector-type air cooking unit which is designed for the ready separation of spilled materials from the heated air.

Another object of this invention is the provision of a convector-type air cooking unit which provides for gravity flow bypass means for the ready separation of spilled material from both the supply and return airflow passages of the unit.

Still another object of the instant invention is to provide outer and inner spill chambers that are interconnected to insure the prompt segregation of spilled material from the heated areas of the unit while also facilitating cleaning thereof.

A still further object of this invention is the provision of an improved convection heat transfer electric range surface unit which provides for optimum thermal efficiency by improving the air mass flow and air direction flow within the unit.

A still further object of this invention is the provision of an improved air convection range surface heater which increases the total heat flux between the surface heater and a vessel thereon.

Yet another object of the present invention is to provide a convector-type air cooking unit in which the air temperature adjacent the heating coils is maintained at or below a maximum temperature.

Still another object of the invention is the provision of a solid state semiconductor control means for a convector-type air cooking unit to prevent over heating of the heater coils from any cause.

A further object of the present invention is to provide a control means for convector-type air cooking unit in which the air temperature is maintained at or below a preselected setting having solid state semiconductor switch means therein for varying the power supply to the heater element in accordance with sensed conditions within the hot air plenum and including means for completely deenergizing the unit.

Other advantages found in the improved range surface heater of the invention are the use of ceramic coatings on the surface components, available in a range of colors made possible because of the lower temperatures of the surface components; reduction in the number of hardware components required for construction of the cooking top; provision for adjustment of the air circulation gap between a utensil and the unit; a unit having low-thennal mass; rapid attenuation of heat flux to the cooking vessel on shutoff of the unit; and whereby uniformity of temperature of the cooking surface is maintained because dynamic means are provided to monitor and control the temperature of the heating medium rather than only the power input.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

FIG. 1 is a fragmentary perspective view of an electric range suitable for use with the invention;

FIG. 2 is a fragmentary perspective sectional view taken along the line 2-2 of FIG. 1 with parts broken away to show in detail one of the convection air cooking units;

FIG. 3 is a fragmentary section taken along the lines 3-3 of FIG. 1; and

FIG. 4 is a diagrammatic view of an electrical circuit constructed in accordance with principles of the present inventron.

In accordance with this invention and with reference to FIG. I, an electric range 10 is illustrated. The range 10 is provided with a range top 12 having a plurality of surface heaters or cooking units such as 14 on the side thereof. Suitable controls such as 16 are provided on a console 18 of the range and adapted for selectively energizing the surface cooking unit 14.

FIG. 1 depicts a novel air convection range surface heater or cooking unit 14 wherein a current of superheated air radially traverses the top of the surface cooking unit from the outer periphery to the center of the unit. As explained in the above-mentioned US. Pat. No. 3,167,638 in its flow the air will transfer its heat very evenly to any utensil placed thereon. The surface of the heating element 14 may be kept clean and bright because the cooking vessel contact area operates at a relatively low temperature, permitting the use of special hightemperature vitreous enamel as a coating for parts of this area. The enamel, more properly called the ceramic coating, remains bright and lustrous during operation and in the case of soiling can be cleaned easily by using kitchen methods. Further, such enamel can be modified to match a wide variety of colors obtainable with conventional vitreous enamels. These factors enhance the styling possibilities with the surface cooking units 14.

In the prior art convector units spilled materials may be allowed to come into prolonged association with the heated air resulting in a possibility of overheating the spillage and a possible blockage of the airflow ducts. Furthermore, many prior art convection heat-transfer designs have a centrifugaltype fan such that the air leaving the fan has a large undesirable tangential velocity component or swirl. It was discovered by applicant that if the air leaving the fan could be redirected radially inwards by some means it would not only improved the airflow through the heat exchanger by eliminating the swirl but would convert the high velocity of the air of a high pressure whereby the desired mass flow of air through the heat exchanger could be maintained. In the following description of the cooking unit 14 of this invention it will be seen how the spillage and air mass flow problems, among others, have been eliminated.

With reference to FIG. 2, a sectional view of the convection air surface heater or cooking unit of this invention is shown comprised of a decorative annular pan rest for pan support ring 20 located on the top surface 12 of the range. The pan support ring 20 which is preferably enamel coated, rests about a circular opening 13 in the range top surface and is outwardly sloped to direct spillage away from the opening. A center baffle rest 22 is provided for downwardly directing the return air and, as seen in FIG. 3, is provided with spaced resilient clip arms 23 for removably retaining the center baffle on the unit at the desired elevation. The upper surface of the center baffle rest can be coated with an enamel finish in the manner such as described above.

In FIG. 2 there is shown a portion of the range top 12 having a surface heating unit aperture 13 adapted to support an outer casing 24 by means of an annular collar member 26 which is fixed to the outer surface of the casing 24 by suitable means such as by welding. The collar 26 has an outwardly directed flange 27 which rests directly on the flat upper surface of the range top 12. The collar 26 supports a circular trim member 28 having an inverted L cross section for overlying the collar 26 and forming a support for the pan support ring 20. A disklike burner top member 30, which also can be coated with enamel, extends radially a spaced distance from the outer ring 20 to a location beneath the center baffle 22 while being provided with a depending outer snap-flange 32 and an inner inverted conical portion 34 terminating in a downwardly directed spout or cylindrical collar 36 (FIG. 3). In this way, the burner top member or disk 30 may lay in snapfitting relationship to underlying portions of the cooking unit now to be described.

Upon removal of the support ring 20, baffle 22 and burner top disk 30 there is exposed a heater coil assembly, generally indicated at 38, annular deflector member 40 and outer support shell 42. The heater coil assembly 38 comprises an upper U-shaped heater guard 44 for supporting the burner top disk 30 in snap-action fashion thereon, a resistance coil 45, and a coil support ring 46 formed from a refractory material such as ceramic or the like which is suitably secured to the heater guard 44 such as by through bolts indicated at 48. With reference to FIG. 3, it will be seen that the coil support ring 46, composed in the preferred embodiment of six individual pie-shaped sections, has a central opening 47 dimensioned for slidable mounting on center return air tube 50 and is retained on the upper flange 52 of the tube 50 to be substantially flush with the upper end thereof. In this manner the heater coil assembly 38 can be removed from the cooking unit casing 24 by simply lifting it vertically upwards thus permitting ease of maintenance and ready access to the internal portions of the unit. Notice that the coil support ring 46 is supported at its outer periphery 49 by a retaining band 54 having a base leg 55 located on the horizontal plate portion 56 of an inner shell member 58. It will be observed that the inner shell 58 has an annular wall portion 59 enclosing a stator assembly generally indicated at 60, the details of which will be explained subsequently.

As viewed in FIG. 3 the support band 54 has an inwardly directed flange 62 in opposed alignment with tube flange 52 and upon which the outer portion of coil support ring 46 rests. Further, the heater guard 44 has a downwardly directed outer rim 64 having a series of closely spaced, generally rectangular apertures 66 (FIG. 2) providing peripheral outlet means for an upper hot air annular plenum or cavity 68, bounded by the coil support ring 46 and spaced heater guard 44. A supply air outlet or annulus 70 is defined by the outer snap-flange 32 of the burner disk 30 and the opposed concentric lip 72 of the pan support ring 20. It will be observed that heater guard rim 64 has a conical ledge 74 designed to nest on the outwardly sloping portion of the support shell 42.

The air deflector member 40 is formed having a substantially right-angle cross section, the vertical leg 76 of which forms an air deflector to direct the substantially horizontal passage of he hot air (solid arrows in FIG. 2) exiting from the plenum apertures 66 upwardly to the supply air outlet 70. The horizontal leg 78 of the deflector 40 has a continuous series of closely spaced apertures or spillage holes 80, illustrated in FIG. 2 of the drawings, located alignment with gravitational flow of spillage entering through annulus 70. The spillage is outwardly directed past upper plenum apertures 66 by the flared end of snap-flange 32. The bypass apertures 80 in turn provide ready flow of spillage to a vertical spill passage 82, defined by the outer support shell 42 and the casing member 24, leading to an outer spill chamber 84 occupying the bottom of the casing.

Thus, when spillage enters the outlet 70 it will be shunted directly into the passage 82 because of the natural force of gravity acting on the substantially greater mass of liquid and solid spilled material. The result is that such spillage is immediately segregated from the hot airflow and passed directly to the outer spill chamber 84. It will further be seen that applicants design allows any spilled solid material of a size greater than bypass holes 80 to be retained by the deflector member 40 without blocking the hot airflow from the upper plenum 68 to the supply air outlet 70. In addition it should be observed that the snap flange 32 of the burner top disk acts, together with the heater guard 44, as a shield to prevent possible direct or straight line radiation from the heater coil 45 upwardly to the cooking utensil.

As seen in FIG. 3 the outer shell 42, comprising upper conical wall portion 86 and vertical wall 88, has an outwardly directed stiffening flange 90 supported directly on a projecting ledge 91 of the inner shell 58.

The outer spill chamber 84 is defined in part by the bottom plate 104 of casing 24 and a pan member generally indicated at 92 whose upwardly extending peripheral sidewall 93 ends in an outwardly turned continuous rim 94 resting directly on a plurality of adjusting screws, one of which being indicated at 96. Centered within the flat bottom wall 95 of underlying pan 92 is an opening 98 for reception of the central tubular support member 100 the purpose of which will be discussed hereinafter. The adjusting screws 96 are threadly supported in an O-ring member 102, suitably affixed as by welding to the outer casing 24, for supporting the bottom plate 104 portion of the outer casing 24.

The upper stern 106 of the adjusting screw 96 extends upwardly through vertically aligned apertures in a channel section 108 welded or otherwise suitably affixed to the casing 24. A bracket member 110 has its vertical leg 111 spot welded to the face of the channel section 108 while its inwardly projecting leg 112 serves as the hinge support for the fixed portion of a flapper valve member 114. The valve 114 seals a tubular or spill trap drain 116 communicating with an offset aperture 118 located in the bottom wall 95 surface of the pan member 92. The flapper valve 114, formed of a flexible material such as Teflon (a polytetrafluroethylene plastic), interconnects an inner spill chamber 120 defined by the pan 92 with the underlying outer spill chamber 84. The inner spill chamber 120 receives spilled material from an air return inlet or annulus, indicated at 122, and defined by the center baffle rest 22 and the conical portion 34 of burner top disk 30. In operation, flapper valve 114 is normally closed by the air pressure difference between chambers 84 and 120 and open only when the weight of the ingested liquid entering through inlet 122 is sufficient to open the valve 114 allowing the material to pass into the outer spill chamber 84.

FIG. 2 shows how spillage (dashed arrows) entering through the air return inlet 122 is directed inward and downward into the inner return air passage 124 provided by the tube 50. The lower end of the tube 50 telescopes into a depending housing 126 of a centrifugal impeller means, generally indicated at 128, while defining a return air corridor 127. The depending housing 126 functions as an intermediate spillage trap having closely spaced windowlike openings 130 communicating with the inner spill chamber 120 thus allowing any spillage to readily separate from the return airflow in passage 124 (solid arrows) by gravitational action and cascade into the surrounding chamber 120.

Turning now to the drive arrangement it can be seen that the impeller 128 is connected via a mounting hub 131, extending downwardly from its housing 126, to a vertical shaft 134. The mounting hub 131 has a head portion 132 fixedly attached to the housing 126 ad a stem portion 133 journaled in the tubular support 100 in any suitable manner such as by seating in a cup-type bearing 136. The hub 131 is fastened to the hollow shaft 134 such as by a key 138. Motive means provides the drive for the impeller 128 and is arranged so as to be concealed beneath the range top. The motive means 140 in the instant embodiment is a standard AC universal, l 15 volt, electric motor. The motor drive shaft 142 is slip fitted within the tubular shaft 134 and is retained in driving relationship therein by means of a threaded stop bolt 144 on a fiat of the shaft. The motor 140 is bracketed to the casing bottom plate 104 by collar 146 and held by suitable securing means such as machine bolts shown at 148.

A pedestal member 150 is fastened to the bottom plate 104 by means of bolts 148 which extend downwardly to support the motor collar 146. The pedestal member has a central upstanding bore 151 for receiving the lower end of the tubular support 100 in sliding-fit fashion. Molded cup packing seal 152 surrounds the shaft 134 and is retained in place by pressure ring nut 154 allowing sealed rotary motion of the shaft 134 for driving the impeller in a counterclockwise direction.

No difficulties have been experienced in the disclosed embodiment with over heating of the motor 140 as a result of the surface unit operation. However, it should be understood that motor design operation within higher ambient temperatures is well within the skill of a mechanic. It should be further understood that a single motor could be used to drive impellers in the other air convection surfaces heating units such as shown at 17 in FIG. l--the motor being energized whenever any one of the surface heaters is energized.

The lower pan assembly 92 is closed by an annular cover member 156 comprising an upper cover sheet 158 and a spaced lower cover sheet 160 being heat-insulated therefrom by a suitable material such as a batt of fibrous glass 162 compressed therebetween to minimize the heat transfer into the pan 92. Note that the cover member 156 has a central opening 159 and includes a sealing gasket 161 formed of silicon rubber, of the like, to thermally isolate the cover from the housing 126 while sealing the return airflow passage. Cover sheets 158, 160 terminate in a double flanged edge 16 which seats on the continuous rim 94 of the pan 92, this assembly in turn supporting projecting ledge 91 of the inner shell member 58. As seen in FIG. 3, the ledge 91 together with the double flange 163 of the cover member are affixed to the pan rim 94 by suitable means such as rivets 164 and a sealing gasket 165 is placed therebetween for sealing air within inner shell duct 166 and minimizing heat conduction between the shell structure and the pan 92. It should be noted that the space between outer shell 42 and inner shell 58 is also provided with a suitable heat insulation material 167 such as a batt fibrous glass to further reduce the heat loss from the unit 14 and minimize the requirements for preheating the unit.

As best seen in FIG. 3, the stator 60 has generally radially directed curved vanes 168 fixedly supported between horizontal plate 56 and the underlying stator plate 169. The stator vanes 168 are suitably curved in a complemental manner to the forwardly curved impeller blades 170 shown sandwiched between an impeller plate 171 ad the horizontal plate 172 forming a continuation of impeller housing 126. It will be noted that the term forwardly curved relates to the fact that the blades 170 are curved toward the direction of impeller rotation which in the instant embodiment is counterclockwise. Applicants air circulation design enables the stator vanes 168 to change the flow path of the air from the fan blades 170, as viewed by the solid arrows in FIG. 2, from a high-velocity tangential swirl to a radially inward flow of the lower velocity but higher pressure. The lower stator support plate 169 is affixed to the return air tube 50 by suitable means such as welding to a lower flange 174 of the tube 52.

Explaining now the heat exchange portion of the unit the continuous resistance coil 45 of known spiral type, is held in a spiral U-shaped groove 177 in the upper face of the coil support ring 46. The ceramic ring has passage means preferably comprising a continuous series of closely spaced, counterbored air passage holes shown at 178 communicating with the spiral groove 177. In this way the airflow upon exiting from the stator vanes 168 moves upwardly into a lower plenum or cavity 179 and is heated by traveling vertically through holes 178 past the resistance coil 45 and into the upper plenum 68. Applicants new heat exchange design permits maximum efficiency in the transfer of heat exchange energy to the air through balanced distribution to the coil. Further it allows for a pressure differential between lower plenum 179 and upper plenum 68 thereby increasing the mass flow of air to the cooking utensil.

Power is delivered to the coil 45 by means of plug adapt 180 which extends into casing 24 through tube 182 shown protruding into vertical passage 82. Leads 183 extend through plug 180 and insulation sleeve 184 terminating in a pair of contactor receiving members 186. A pair of spade connectors 188 are provided on the underside of ceramic ring 46 for ready in sertion in the two receiving members 186. This arrangement provides an insulated plug-in connection to insure against short circuiting and shock hazards due, for example, to possible salt water spillage into the air supply outlet. The plug-type connectors 186, 188 provide for connecting the coil 176 across a 230-volt AC Edison power supply to obtain various wattage outputs, as well understood in the art.

Considering now the loaded condition of the surface unit 14, FIG. 3 illustrates a recirculating air system. With the energization of the motor and the heating coil 45 and with a utensil such as pan 190 (shown in phantom) resting on the support ring 20 a closed air circulation space or gap 192 will be formed between the bottom 194 of the pan and the burner top disk 30. Thus, as previously indicated by means of the solid arrows of FIG. 2 air is forced by the impeller blades into the air supply duct 166 the air turns substantially and enters between the stator vanes 168. The stator vanes 168 change the flow of the air from a high-velocity tangential or cyclonic swirl imparted by the forwardly curved fan blades 170 to a radially inward flow of lower velocity but higher pressure. The decrease in density of the air accompanying the temperature increase in the heat exchange upper plenum 68 requires additional pressure rather than velocity to maintain the desired mass flow of air to the air circulation gap 192.

When it is desired to initially adjust the height of the air circulation gap 192 upon installation of the unit the adjusting screws 96 are rotated by insertion of a screwdriver into the slotted head portion 97 of the screws. Rotation of adjusting screws 96 vertically shifts the inner assembly including the shell members 42 and 56 which in turn vertically adjusts the height of the burner top disk 30 with respect to the plane of a utensil bottom supported on the unit. By adjusting the gap 192 the optimum gap spacing can be attained which is a balance between a large air mass flow provided by a large gap and an increased convective heat-transfer coefiicient dependent upon several factors including increased air velocity and air turbulence provided by a small gap. The optimum gap spacing is therefore that spacing which will maximize the heat transfer flux between the airflow and the utensil. It is to be noted that the adjusting screws are located at 120 spaced intervals and this arrangement ensures proper leveling of the heating unit. The slip fit between the bore 151 and tubular support 100 allows the heating unit assembly to move up and down when the adjusting screws are rotated.

With respect to the heating element the entire air stream is heated by passing vertically upward normal to the principal axis of the coil 45 after exciting from the vertical holes 178 in the coil support plate 46. By virtue of distributing the air vertically past a single section of the coil 45 rather than having the same airflow past successive coil sections, local flow restrictions within the heat exchange area are reduced to improve the airflow through the unit. It has been found that for efficient operation the space between helical or individual spirals of coil 45, wound from round wire, should be at least equal to the wire diameter. The air exists the heat exchange plenum 68 through the rectangular openings 66 in the heater guard 44 and is then directed to the under pan space 192 via the supply air outlet 70. After contact with the pan bottom 194 the air is drawn down by means of inlet 122, into the return air passage 124 of the center of tube 50 and makes a 180 turn upward into air corridor 127 and finally returns to the impelling means.

Referring now to FIG. 3, it will be seen that the heated air is directed substantially vertically through the outlet 70. It has been found that when the heated air emerges from the outlet 70 and is aimed directly at the utensil bottom, requiring the air to change direction before flowing radially in towards the center inlet 122, the convective heat-transfer coefficient relative to the utensil is higher that when the initial direction of the heated air is radially inward. It should be mentioned that the instant design does not require that a cooking utensil be in place on the unit to seal the outer ring 20 because the remainder of the airflow path is sealed.

The external top plate hardware consisting of the trim and pan support ring 20, the burner top disk 30, and the center bafile 22 have been formed from austenitic stainless steel (Type 304) which can also be used to form the interior portion of the unit such as shell members 42 and 58, if desired.

Such stainless steel was selected for its superior hot strength over carbon steel grades. However, it is contemplated that a lower grade of alloy will have sufficient hot strength for such interior portions of the unit while being able to meet the temperatures encountered in the air convection range surface heater of this invention.

While in the subject of the cooking unit hardware it will be observed that the surface hardware portions of the invention are designed in such a manner that it is virtually impossible for a small cooking utensil, a utensil with an irregular shaped bottom, or an improperly positioned utensil to choke off the airflow across the gap 192. This result is achieved in part because the center baffle rest 22 has a convex portion 197 on its clip arms 23 operative to shoulder on the conical area 34 of the burner disk 30. Thus the upper surface of the baffle 30 is maintained in a plane that is vertically spaced from the plane of disk 30 to insure against inadvertent blocking of the gap 192 or inlet 122.

With regard so the design for spill handling, the flow path of liquid spillage passing through the air outlet 70 of the surface unit, as indicated by the line dashed arrows in FIG. 2, will be directed away from the apertures 66 of the heater coil guard by the outwardly turned flange 32 of the burner top 30. The spillage travels downwardly through the apertures 80 in the deflector 40, into the vertical spill passage 82 and finally into the outer spill chamber 84. A shield 198, forming a continuation of tube 182, protects the electrical leads 183 from contact with the spillage.

From the foregoing, it can be appreciated that a unique design for the removal of foreign materials from the recirculating air path has been accomplished by taking advantage of the relatively large density variance between air and the liquid spillage. Food and liquid spillage adjacent the center of the unit will drain into the center air return 122 of the unit as indicated in FIG. 3 where it is separated from the airflow by the natural force of gravity and conveyed into the fan housing 126. Centrifugal force imported to the spillage by the rotation of housing 126 accelerates the movement of spillage through the openings i130 and into inner spill chamber 120. The return airflow is directed up into the return air corridor 127 to the blades 170 of the fan. The outlet of tube 50 is provided with a splash deflector 200 at its discharge end to prevent spillage from entering the corridor 127. The spill trap 116 below pan 92 interconnects the two spill chambers 84 and 120 by means of the flapper valve 114 providing greater spill storage capacity of the unit.

The air convection surface unit incorporates a control system which will provide cyclic off-on operation to control the electrical inputs to the resistance coil 45 at the discretion of the operator. The system also allows over-riding maximum air temperatures control to prevent overheating of the unit. A preferred circuit arrangement for controlling the mean flux heat is illustrated in FIG. 4 of the drawings. A conventional pulsating infinite heat switch of a type shown in U.S. Pat. No. 2,623,137 issued to W. H. Vogelsberg for example, is indicated at 202 including a pair of electrical contacts 204 ad 206 with the contact 206 located on a bimetal cycler 208 thermally actuatable in reference to electrical resistance windings 210. Additional contacts 212 provided for the L2 side of the power line and a conventional signal lamp circuit (not shown). When the operator rotates the control 16 on the control panel of the range from the off position all contacts within the switch 202 close and a current of 230 volts, 60-cycle AC is supplied to the heater coil 45 of the surface unit and 118 volts is supplied to the signal lamp circuit. After a time interval, depending upon the users control setting, the bimetal cycler 208 is sufficiently heated 210 to cause the bimetal to warp and open the circuit through the heater coil 45 after which the bimetal cools and returns to its normal shape thus closing the contacts 204 and 206 against completing the circuit to the heater coil. Thus the switch 202 provides a cyclic off-on operation with stepless variable ratios of off-on time, controlling heat inputs that can vary from 5 percent to 100 percent of rated power.

The overriding maximum air temperature control portion of the circuit allows a new concept for surface burner control by making it possible to monitor and control the temperature of the heating medium itself rather than only regulation of the power input. The preferred embodiment of the invention is designed as a conventional 230-volt unit of sufficient power (3500 watts) to produce a rapid warmup and thereafter be throttled back. The throttling action is based on limiting the maximum air temperature in the upper plenum 68 of the heat exchange section such that a maximum temperature cutout overrides s the other control inputs.

As seen in FIG. 3 the control system includes an electric temperature sensor such 4) conventional thermocouple probe indicated at 214 that is supported in the ceramic ring 46 of the unit to sense the air temperature in the upper hot air plenum 68. While the description will be concerned with an electrical control and specifically a thermoelectric type sensing element it will be understood that the instant temperature control system could be modified using resistance elements such as thermistors or resistance bulbs. In addition differential expansion or fluid expansion type thermostats could also be employed without departing from the scope of the invention. The thermocouple 214 is wired directly into a sensitive relay 216 (FIG. 4) having contacts 217 which open or close the gate circuit of a power supply switch which is the preferred embodiment is a solid state silicon controlled rectifier bank generally indicated at 218. The voltage generated in thermocouple 214 acts as a control signal which operates the silicon controlled rectifier bank 218 for supplying power to the heaters resistance coil 45.

The thermocouple 214 functions to open the relay 216 at a preselected level determined by the setting of the potentiometer 220 inserted in series with the thermocouple 214 and he relay 216. In the disclosed unit the potentiometer is adjusted to limit the air temperature in plenum 68 to a maximum of 1,100 F.

As seen in FIG. 4 the rectifier bank 218 is representatively illustrated as being a pair of oppositely facing silicon controlled rectifiers 222, 224 which are actuated by the induced voltage in their associated resistors 226 and 228, respectively, and which are conductive for a predetermined period of the full waveform of the AC power supply across electrical supply lines L1 and L2. The resistor 229 functions as a voltage divider in the circuit.

The rectifier 222 has its anode connected to the terminal 230 and the rectifier 224 has its anode connected to the terminal 232 while gate electrodes 234, 236 are provided respectively with each rectifier. The gate electrode 234 is connected to the cathode of a diode 238 while the gate electrode 236 s connected to the cathode of a diode 240. If the current flow occurs through diode 238 the voltage drop thereacross will cause the gate electrode 234 to be positive with respect to the cathode of the rectifier 222 and the rectifier will conduct. On the next half-cycle current flows through conduit diode 240 the voltage drop across the diode 240 will be applied between the gate electrode 236 and the cathode of the rectifier 224 causing the gate 236 to be positive with respect to the cathode and the rectifier 224 will conduct. Thus, either the rectifier 222 or the rectifier 224 is rendered conductive depending on the direction of current flow between the terminals 230 and 232.

The state of the control rectifiers 222, 224 determines whether the circuit operates, and the direction in which it operates. Thus, when the contacts 217 of relay 216 are opened by the output voltage from the thermocouple 214, induced by the air temperature in plenum 68 exceeding l,l0O F being fed directly to the relay 216 neither of the rectifiers 222, 224 is conductive and the power in the circuit is throttled. The gates 234, 236 have to be energized by a very small current in each half-cycle, or they will not conduct. The thermocouple-relay-rectifier bank system of applicants invention responds readily to the overheating of the heater coil 45 from any cause.

The normal operation of the maximum air temperature control circuit of FIG. 4 is to throttle the power input at the silicon controlled rectifier bank 218. If the rectifier bank fails or is shorted out for any reason, a second relay 242 and associated potentiometer 244 may be connected in parallel with the regular maximum temperature limiting relay 216, and can be adjusted to react if the air temperature in plenum 68 reaches a critical level such as 1,250 F. This second relay 242 can be designed to activate another set of relays (not shown) which will completely deenergize all the cooking units of the range.

In cooking, it is often desirable to stop the effective heat flux very quickly. The effective heat flux in applicants convectiontype surface unit is closely related to the air mass flow and the fan speed. As the motor used in he preferred embodiment is of the type having low-friction ball bearings. the motor fan inertia forces will keep the fan rotating for as long as 30 seconds after the motor has been deenergized. Under these circumstances, the quick-stop" potential of an air convection unit will not be fully realized. Applicants surface unit motor therefore can be provided with an automatic dynamic braking control circuit (not shown) of conventional design by cutting off AC power to the motor and applying DC current across the stator windings. The result is a more rapid attenuation of heat flux to the cooking utensil or shutoff which is notably superior to conventional radiant coil surface units.

It will be noted that suitable controls may also be provided to hold the temperature of the cooking utensil, a so-called pan control. Thus, a second thermocouple probe 246 is shown located in the center bafi'le rest 22 to enable a modified unit to sense the temperature of the vessel 190. By means of the thermocouple 246 the range is able to control the power input according to the demand for maintaining the utensil at a preset temperature. The thermocouple can be wired to a suitable thermostatic switch with conventional temperature indicator markings therein (not shown) located in the front control panel 18 to control the power input to the heater coil 45.

ln view of the foregoing, it will be appreciated that an improved air convection surface heater has been devised for an electric range having a unique airflow system for optimum heat flux transfer to the cooking utensil by increasing the air mass flow and thereby increasing the convective heat-transfer coefficient for the system. A major improvement of the present invention is the uniformity of temperature distribution to the cooking surface and the precision of temperature control. Further, by providing a design allowing for the heat-insulation of the unit applicants have minimized the flow of heat from the heat exchange area to the adjacent portions of the surface heater l4 and range top. The heat insulation barriers 162 and 167 together with the low'thermal mass of applicants unit and the heat-breaker structure incorporating the insulating gaskets 161 and 165 will minimize the requirements for preheating the unit.

Applicants improved cooking unit also markedly reduces the heat flow or output to the cooking utensil on deenergization of the heater-fan combination by means of the heater coil 45 not only being shielded by members 30 and 44 to prevent direct or straight line radiation to the utensil bottom but by providing intervening plenum 68 as well. By designing the plenum 68 in overlying relation to the heater coil the instant invention eliminates the need for heat sink material, such as heat insulation, being located intermediate the air circulation space 192 and the heater coils 45. These and other advantages readily appreciated by one skilled in the art are all embodied in the improved air convection range surface heater of this invention.

While the embodiments of the present invention as herein disclosed, constitute preferred forms, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. A surface cooking unit for an electric range comprising range top means having an opening therein, means in said opening for supporting a utensil on the range top, disk means spaced slightly below the plane of a utensil bottom supported on said supporting means to define a planar air circulation space in said range top means immediately below said utensil bottom, said air circulation space closable at least partially by a utensil supported thereon, a casing member depending from said range top, pan means providing an inner spill chamber located in spaced relation within said casing member and underlying said disk means in space relation thereto, said pan means cooperating with said casing to provide an outer spill chamber therebetween, electrical heating means, means in said unit for supporting said electrical heating means in spaced relation intermediate said disk means and said pan means whereby upper and lower plenums are defined respectively above and below said heating means, supply air outlet means connecting said upper plenum with said air circulation space, vertical passage means connecting said supply air outlet space for heating said utensil, with said outer spill chamber, central air passage means in the form of a central vertical tube having its upper end in communication with said air circulation space and its lower end extending vertically below said air impelling means into said pan means to thereby bypass said heating means and pass spillage directly from said return air inlet means into said inner spill chamber, air return inlet means connecting said air circulation space with said central air passage means which in turn is connected to said lower plenum, air impelling means intermediate said pan means and said lower plenum for circulating air between said heating means and said air circulation space for heating said utensil, and said central air passage means communicating with said inner spill chamber whereby spillage on said unit entering said return air inlet means and said supply air outlet means will pass into said inner and outer spill chambers respectively and bypass said heating means.

2. The surface cooking unit of claim 1, wherein valve means are provided for interconnecting said inner spill chamber and said outer spill chamber said valve means being normally closed in response to the differential air pressure between said inner and outer spill chambers, and said valve means being automatically opened by spillage in said inner spill chamber.

3. The surface cooking unit of claim 1, wherein said impelling means includes a cup-shaped depending fan housing telescopically received in space relation in said pan means, said lower end of said central tube concentrically spaced within said depending housing defining return air corridor means therebetween for supplying return air to said impelling means, said depending fan housing having aperture means spaced vertically below the lower end of said central tube whereby the flow of return air is redirected vertically upwardly to said impelling means while allowing gravitational flow of the spillage into said inner spill chamber.

4. The surface cooling unit of claim 1 wherein air deflecting means are positioned between said upper plenum and said vertical outer passage means, and air deflecting means having an upstanding air deflecting portion redirecting the heated airflow from said upper plenum from a substantially horizontal outward flow to a substantially vertically upward flow.

5. The surface cooking unit of claim 4 wherein said air deflecting means having aperture means underlying said air supply outlet allowing gravitational flow of spillage into said vertical passage to whereby readily separate the spillage from the heated airflow.

6. Surface cooking unit for an electric range comprising range top means in said opening therein, means in said opening for supporting a utensil on the range top, disk means spaced slightly below the plane of the utensil bottom to define a planar air circulation space in said range top means immediately below the utensil bottom, a casing member depending from said range top surrounding said cooking unit, said casing member having concentrically spaced pan means therein underlying said disk means defining an outer spill chamber between said pan means and said casing, said pan means defining an inner spill chamber, concentrically spaced shell means in said casing supported on said pan means, central tube means depending from said disk means and concentrically spaced within said shell means establishing supply air duct means therebetween, said shell means ans said tube means cooperatively supporting electrical heating means in spaced relation below said disk means thereby defining an upper plenum intermediate said disk means and heating means, outer supply air outlet means connecting said upper plenum with said air circulation space, vertical passage means defined intermediate said casing member and said shell means connecting said supply air outlet means with said outer spill chamber, air impelling means supported in said supply air duct means in spaced relation below said heating means operative to circulate air between said heating means and said air circulation space for heating the utensil, said central tube means providing a return air passage connecting said air circulation space with said supply air duct means, stator means concentrically supported in said supply air duct means in overlying relation to said impelling means defining a lower plenum intermediate said stator means and said heating means, said lower plenum communicating with said supply air duct means via said stator means whereby air exiting from said stator means is directed radially inwardly to said lower plenum means.

7. The surface cooking unit of claim 6 wherein said impelling means includes a depending housing concentrically spaced within said pan means, and said central tube means being telescopically received in spaced relation within said depending housing thereby defining a vertical return air corridor therebetween connecting said return air passage with said impelling means.

8. The surface cooking unit of claim 7 wherein said depending housing has aperture means therein providing communication between said return air passage and said inner spill chamber permitting spillage from a cooking operation to bypass said heating means and pass from said air return passage and into said inner spill chamber.

9. The surface cooking unit of claim 6 wherein said heating means comprises an electrical resistance coil, a coil support ring and an annular heater guard having an inverted channelshaped cross section disposed in overlying concentric relation to said coil support ring said coil support ring and said heater guard forming said upper plenum, said heater guard having peripheral air outlet means connecting said upper plenum with said vertical passage means, and deflector means in said vertical passage means operative to deliver the heated air from said upper plenum to said outer supply air outlet means, and aperture means in said deflector means located below said peripheral air outlet means to provide ready spillage segregation from the heated air.

10. The surface cooking unit of claim 6 wherein said pan means having a heat-insulating cover member, said cover member having a central opening therein for passage of said depending housing therethrough, and gasket means substantially sealing said cover member from said depending housing to provide a thermal break in the heat patch between said pan means and the heated air.

11. The surface cooking unit of claim 6 wherein control means are provided for throttling the power input to said electrical heating means to limit the maximum air temperature in said upper plenum of said unit.

12. A surface heating unit for an electric range comprising range top means having an opening therein, means in said opening for supporting a utensil on the range top, a burner disk spaced a predetermined distance below the plane of a utensil bottom supported on said supporting means to define a planar air circulation space in said range top means immediately below said utensil bottom, said air circulation space closable at least partially by a utensil supported thereon, an

outer casing member depending from said range top, a heater coil assembly in said casing member including a coil support ring having a central opening a center return air tube said support ring retained on an upper flange of said return air tube so as to be substantially flush with the upper end thereof, said return air tube flange supporting said heater coil assembly in s aced relation below said burner disk whereby an upper p enum is defined between said burner disk and said heater coil assembly, said coil support ring having a convolute U- shaped groove in its upper face, a continuous spiral resistance coil supported in said convolute groove, vertical air passages providing communication between said groove and a lower plenum whereby air from said lower plenum flows through said air passages and is heated by said heating coil as it flows into said upper plenum, supply air outlet means connecting said upper plenum with said air circulation space, air impelling means in the form of a centrifugal fan having forwardly curved blades located in spaced relation below said heater coil assembly, stator means in the form of stationary forwardly curved vanes fixedly supported between parallel horizontal plates defining said lower plenum between said stator means and said heater coil assembly, return air passage means connecting said air circulating space with said lower plenum, said centrifugal fan operative for recirculating air serially from said lower plenum, through said heater coil assembly to said upper plenum, and then to said air circulation space for heating the utensil, whereby the flow of air from said centrifugal fan blades to said stator vanes is changed from an outwardly directed tangential swirl to a radially inward and thence upwardly directed flow of air into said lower plenum such that he resultant pressure in said lower plenum is increased to thereby provide a sufficient air mass flow to said air circulation space to maximize the convection rate of heat transfer along the plane of a utensil bottom supported on said surface cooking unit, and means for adjusting said burner disk relative to the plane of a utensil bottom supported on said supporting means to change said predetermined distance thereby to vary the air mass flow to said air circulation space.

13. The surface heating unit of claim 12 including an alternating current power supply for energizing said resistance coil, said power supply having first and second conductors, bimetal switch means connected in series circuit with said first conductor for controlling said resistance coil in accordance with a predetermined operating temperature selection, temperature sensing means in said upper plenum for controlling said power supply in accordance with with a preselected maximum temperature in said upper plenum, said temperature sensing means including ta thermocouple sensing device for generating a voltage proportional to said preselected maximum temperature in said upper plenum, semiconductor switch means in the form of a silicon-controlled rectifier bank in series with said power supply, said rectifier bank including a pair of opposed silicon-controlled rectifiers in parallel with said second conductor, each of said silicon-controlled rectifiers including a gate electrode, a diode connected between said power and each said gate electrode whereby each said silicon-controlled rectifier is rendered conductive during one half-cycle of the current from said power supply, relay contact means in series with said temperature sensing means interposed between said silicon-controlled rectifiers wherein said relay is operative to open and close the gate circuit of said silicon controlled rectifiers on each half-cycle of current from said power supply and potentiometer means in series with said relay contact means for adjusting the flow of current through said relay contact means to limit the air temperature in said upper plenum to the desired maximum value.

zs g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,632, 980 Dated ,Ianuary 4, 1972 Invenwfls) James R. Hornaday, Jr., Edwin J. Miller, Charles w.

vigor It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' '1 r- In the Specification:

Column 2, line 42, "applicant" should read applicants line 45, "of" (second occurrence) should read to Colurm 3, line 49, "he" should readthe line 53, after "located" insert in---: line 68, "cant's" should read cants' Column 4, line 13, "threadly" should read threadably line 35, "open" should read opens line 55, "ad" should read and Column 5, line 17, "of" should read or line l9, "16" should read 163 line 38, "ad" should read and Column 6, line 27, "56" should read 58 line 44, "exciting" should read exiting Column 7, line 22, "line" should read long line 41, "'130" should read 130 line 53, "temperatures" should read temperature line 58, "ad" should read and line 72, "against" should read again Column 8, line 13, "4) should read as a line 26, "is" should read in line 34, "he" should read the line 50, "s" should read is line 55, delete "conduit" and insert the Column 9, line 14, "he" should read the line 554, delete "o".

P0405) UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent No. 3I Q33I 980 Dated January ll; 1912 lnventgrw) James R. Hornaday, Jr., Edwin J. Miller, (Sharles W.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as. shown below:

Pa e 2 In the Claims:

Claim 1, line 15, delete "space" and. insert means (CoLlO) line 16, delete "for heating said utensil, "a

(C01. 10) line 53, delete "and" and insert said Claim 5,

(Col. 10) line 61, "whereby" shoul retail-- thereby Claim line 64, cancel "in said", last 5 occurrence and. inserti -yi 2%; havmg an """s Claim 10, I 7

(Col. 11) line 54, "patch" should read path Claim 12,

(C01. 12) line 30, "he" should read the Claim 13,

(C01. 12) line 46, delete "with" (second occurrence).

line 48, "ta" should read a line 55, after "power" insert supply Signed and sealed this 1st day of August 1972.

(SEAL) L Attest: .J

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. A surface cooking unit for an electric range comprising range top means having an opening therein, means in said opening for supporting a utensil on the range top, disk means spaced slightly below the plane of a utensil bottom supported on said supporting means to define a planar air circulation space in said range top means immediately below said utensil bottom, said air circulation space closable at least partially by a utensil supported thereon, a casing member depending from said range top, pan means providing an inner spill chamber located in spaced relation within said casing member and underlying said disk means in space relation thereto, said pan means cooperating with said casing to provide an outer spill chamber therebetween, electrical heating means, means in said unit for supporting said electrical heating means in spaced relation intermediate said disk means and said pan means whereby upper and lower plenums are defined respectively above and below said heating means, supply air outlet means connecting said upper plenum with said air circulation space, vertical passage means connecting said supply air outlet means with said outer spill chamber, central air passage means in the form of a central vertical tube having its upper end in communication with said air circulation space and its lower end extending vertically below said air impelling means into said pan means to thereby bypass said heating means and pass spillage directly from said return air inlet means into said inner spill chamber, air return inlet means connecting said air circulation space with said central air passage means which in turn is connected to said lower plenum, air impelling means intermediate said pan means and said lower plenum for circulating air between said heating means and said air circulation space for heating said utensil, and said central air passage means communicating with said inner spill chamber whereby spillage on said unit entering said return air inlet means and said supply air outlet means will pass into said inner and outer spill chambers respectively and bypass said heating means.
 2. The surface cooking unit of claim 1 wherein valve means are provided for interconnecting said inner spill chamber and said outer spill chamber said valve means being normally closed in response to the differential air pressure between said inner and outer spill chambers, and said valve means being automatically opened by spillage in said inner spill chamber.
 3. The surface cooking unit of claim 1 wherein said impelling means includes a cup-shaped depending fan housing telescopically received in space relation in said pan means, said lower end of said central tube concentrically spaced within said depending housing defining return air corridor means therebetween for supplying return air to said impelling means, said depending fan housing having aperture means spaced vertically below the lower end of said central tube whereby the flow of return air is redirected vertically upwardly to said impelling means while allowing gravitational flow of the spillage into said inner spill chamber.
 4. The surface cooling unit of claim 1 wherein air deflecting means are positioned between said upper plenum and said vertical outer passage means, and air deflecting means having an upstanding air deflecting portion redirecting the heated airflow from said upper plenum from a substantially horizontal outward flow to a substantially vertically upward flow.
 5. The surface cooking unit of claim 4 wherein said air deflecting means having aperture means underlying said air supply outlet allowing gravitational flow of spillage into said vertical passage to whereby readily separate the spillage froM the heated airflow.
 6. Surface cooking unit for an electric range comprising range top means having an opening therein, means in said opening for supporting a utensil on the range top, disk means spaced slightly below the plane of the utensil bottom to define a planar air circulation space in said range top means immediately below the utensil bottom, a casing member depending from said range top surrounding said cooking unit, said casing member having concentrically spaced pan means therein underlying said disk means defining an outer spill chamber between said pan means and said casing, said pan means defining an inner spill chamber, concentrically spaced shell means in said casing supported on said pan means, central tube means depending from said disk means and concentrically spaced within said shell means establishing supply air duct means therebetween, said shell means and said tube means cooperatively supporting electrical heating means in spaced relation below said disk means thereby defining an upper plenum intermediate said disk means and heating means, outer supply air outlet means connecting said upper plenum with said air circulation space, vertical passage means defined intermediate said casing member and said shell means connecting said supply air outlet means with said outer spill chamber, air impelling means supported in said supply air duct means in spaced relation below said heating means operative to circulate air between said heating means and said air circulation space for heating the utensil, said central tube means providing a return air passage connecting said air circulation space with said supply air duct means, stator means concentrically supported in said supply air duct means in overlying relation to said impelling means defining a lower plenum intermediate said stator means and said heating means, said lower plenum communicating with said supply air duct means via said stator means whereby air exiting from said stator means is directed radially inwardly to said lower plenum means.
 7. The surface cooking unit of claim 6 wherein said impelling means includes a depending housing concentrically spaced within said pan means, and said central tube means being telescopically received in spaced relation within said depending housing thereby defining a vertical return air corridor therebetween connecting said return air passage with said impelling means.
 8. The surface cooking unit of claim 7 wherein said depending housing has aperture means therein providing communication between said return air passage and said inner spill chamber permitting spillage from a cooking operation to bypass said heating means and pass from said air return passage and into said inner spill chamber.
 9. The surface cooking unit of claim 6 wherein said heating means comprises an electrical resistance coil, a coil support ring and an annular heater guard having an inverted channel-shaped cross section disposed in overlying concentric relation to said coil support ring, said coil support ring and said heater guard forming said upper plenum, said heater guard having peripheral air outlet means connecting said upper plenum with said vertical passage means, and deflector means in said vertical passage means operative to deliver the heated air from said upper plenum to said outer supply air outlet means, and aperture means in said deflector means located below said peripheral air outlet means to provide ready spillage segregation from the heated air.
 10. The surface cooking unit of claim 6 wherein said pan means having a heat-insulating cover member, said cover member having a central opening therein for passage of said depending housing therethrough, and gasket means substantially sealing said cover member from said depending housing to provide a thermal break in the heat path between said pan means and the heated air.
 11. The surface cooking unit of claim 6 wherein control means are provided for throttling the power input to said electrical heating means tO limit the maximum air temperature in said upper plenum of said unit.
 12. A surface heating unit for an electric range comprising range top means having an opening therein, means in said opening for supporting a utensil on the range top, a burner disk spaced a predetermined distance below the plane of a utensil bottom supported on said supporting means to define a planar air circulation space in said range top means immediately below said utensil bottom, said air circulation space closable at least partially by a utensil supported thereon, an outer casing member depending from said range top, a heater coil assembly in said casing member including a coil support ring having a central opening a center return air tube said support ring retained on an upper flange of said return air tube so as to be substantially flush with the upper end thereof, said return air tube flange supporting said heater coil assembly in spaced relation below said burner disk whereby an upper plenum is defined between said burner disk and said heater coil assembly, said coil support ring having a convolute U-shaped groove in its upper face, a continuous spiral resistance coil supported in said convolute groove, vertical air passages providing communication between said groove and a lower plenum whereby air from said lower plenum flows through said air passages and is heated by said heating coil as it flows into said upper plenum, supply air outlet means connecting said upper plenum with said air circulation space, air impelling means in the form of a centrifugal fan having forwardly curved blades located in spaced relation below said heater coil assembly, stator means in the form of stationary forwardly curved vanes fixedly supported between parallel horizontal plates defining said lower plenum between said stator means and said heater coil assembly, return air passage means connecting said air circulating space with said lower plenum, said centrifugal fan operative for recirculating air serially from said lower plenum, through said heater coil assembly to said upper plenum, and then to said air circulation space for heating the utensil, whereby the flow of air from said centrifugal fan blades to said stator vanes is changed from an outwardly directed tangential swirl to a radially inward and thence upwardly directed flow of air into said lower plenum such that the resultant pressure in said lower plenum is increased to thereby provide a sufficient air mass flow to said air circulation space to maximize the convection rate of heat transfer along the plane of a utensil bottom supported on said surface cooking unit, and means for adjusting said burner disk relative to the plane of a utensil bottom supported on said supporting means to change said predetermined distance thereby to vary the air mass flow to said air circulation space.
 13. The surface heating unit of claim 12 including an alternating current power supply for energizing said resistance coil, said power supply having first and second conductors, bimetal switch means connected in series circuit with said first conductor for controlling said resistance coil in accordance with a predetermined operating temperature selection, temperature sensing means in said upper plenum for controlling said power supply in accordance with with a preselected maximum temperature in said upper plenum, said temperature sensing means including ta thermocouple sensing device for generating a voltage proportional to said preselected maximum temperature in said upper plenum, semiconductor switch means in the form of a silicon-controlled rectifier bank in series with said power supply, said rectifier bank including a pair of opposed silicon-controlled rectifiers in parallel with said second conductor, each of said silicon-controlled rectifiers including a gate electrode, a diode connected between said power supply and each said gate electrode whereby each said silicon-controlled rectifier is rendered conductive during one half-cycle of the current from said power supplY, relay contact means in series with said temperature sensing means interposed between said silicon-controlled rectifiers wherein said relay is operative to open and close the gate circuit of said silicon controlled rectifiers on each half-cycle of current from said power supply and potentiometer means in series with said relay contact means for adjusting the flow of current through said relay contact means to limit the air temperature in said upper plenum to the desired maximum value. 